A binary watch has 4 LEDs on the top which represent the hours (0-11), and the 6 LEDs on the bottom represent the minutes (0-59).
Each LED represents a zero or one, with the least significant bit on the right.
For example, the above binary watch reads “3:25”.
Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent.
Example:
Input: n = 1
Return: ["1:00", "2:00", "4:00", "8:00", "0:01", "0:02", "0:04", "0:08", "0:16", "0:32"]
Note:
The order of output does not matter.
The hour must not contain a leading zero, for example “01:00” is not valid, it should be “1:00”.
The minute must be consist of two digits and may contain a leading zero, for example “10:2” is not valid, it should be “10:02”.
分别对小时、分钟两个部分进行回溯,然后将结果进行组合。
public class Solution {
int hour[] = new int[] { 1, 2, 4, 8 };
int minu[] = new int[] { 1, 2, 4, 8, 16, 32 };
public List<String> readBinaryWatch(int num) {
List<String> res = new ArrayList<String>();
for (int i = 0; i <= num; i++) {
dspCombination(hour, i, 0, true);
dspCombination(minu, num - i, 0, false);
for (int m : hs) {
for (int n : ms) {
if (n == 0)
res.add("" + m + ":00");
else {
if (n / 10 == 0)
res.add("" + m + ":0" + n);
else
res.add("" + m + ":" + n);
}
}
}
hs.clear();
ms.clear();
}
return res;
}
List<Integer> list = new ArrayList<Integer>();
List<Integer> hs = new ArrayList<Integer>();
List<Integer> ms = new ArrayList<Integer>();
private void dspCombination(int[] nums, int k, int level, boolean flag) {
if (list.size() == k) {
int sum = 0;
for (int num : list) {
sum += num;
}
if (flag) { // 当前是hour
if (sum <= 11)
hs.add(sum);
} else {
if (sum <= 59)
ms.add(sum);
}
return;
} else if (list.size() > k) {
return;
} else {
for (int i = level; i < nums.length; i++) {
list.add(nums[i]);
dspCombination(nums, k, i + 1, flag);
list.remove(list.size() - 1);
}
}
}
}